The invention relates to a process for the production of a ball game racket frame, in particular a tennis racket, squash racket or badminton racket frame, with a head and a shaft, in which at least the head and/or the shaft is made of a fiber-reinforced plastic as a hollow body.
Frames for ball rackets, e.g., for tennis, squash and badminton, were produced in the past mostly of wood. A wooden frame for a ball racket had a complicated design that comprised many layers and laminates that had to be connected to one another. The production of such a frame was a difficult and time-consuming process. Such wood designs show favorable playing features.
Recently, mainly new materials, such as metal and reinforced plastics, have been used for racket construction.
Because of the greater specific weights of these materials, it became necessary to develop frame embodiments or designs that are hollow, because otherwise the weight of the racket would become unacceptably high. Such hollow frame designs can, in the case of metals, be made by a drawing or extrusion process. In the case of reinforced plastic materials, hollow body designs can be achieved by lamination processes and encapsulation of a light core material, e.g., plastic foam, or by using a hollow molding process, e.g., the blow molding process.
From AT-PS 381 864 there has already become known a process of the above-mentioned type in which supports, made integrally with a racket head designed as a hollow body, were molded onto the walls. The formation of the hollow spaces was performed here according to conventional processes and, according to this proposal, a core that melts at low temperature is used during injection molding. Also from EP-A-188 127 it has already become known to perform a so-called melting core process in which the core, also brought in to form the hollow space, was melted out by heating after the racket wall sets. This process is very expensive, since the low-temperature melting core must be premolded.
As is usual with injection molding of molded parts, conventional injection molding processes can be performed preferably with materials that are brought in as melts. Such materials, as they are usually used for injection molding processes, are known. But ball rackets produced with injection molding processes do not exhibit the great rigidity and strength at sufficiently low weight that are desired for a really high quality racket. The known plastic rackets are molded with a head part that is not hollow, and its strength to weight ratio or rigidity to weight ratio is lower than is desired for highly qualified ball game rackets. Thus, these ball game rackets are less rigid and less strong than those made of a combination of continuous yarn or fiber materials and a matrix.
In any case, with the previously known injection molding processes for the production of ball racket frames, expensive molds are used to be able to inject both the supports and crosspieces necessary for stability. Recovery of the low temperature melting core represents an additional expenditure of time. In all these production processes, crosspieces remain between the walls, which essentially additionally offer only limited strengths since, viewed structurally, the crosspieces lie in the neutral zone of the frame and thus, only as additional weight, can exert a detrimental influence on the swing behavior or balance of the frame.
From DE-A1 34 16 377 there has become known another injection molding process for tennis racket frames in which, to achieve the necessary stability, especially expensive measures were taken. In particular, the process known from the DE-A1 34 16 377 consists in that a majority of injection molded parts are to be connected to one another by welding, bonding and optionally screwing. Ribs, crosspieces, supports and connections are required by the process in these conventional injection molding processes.
In all previously known injection molding processes, it was possible to achieve a change in the wall thickness essentially only in that several molds were worked with and several parts were later connected to one another. When crosspieces with perforations for accepting the strings are molded on, the final position of the holes provided for the strings are already established in the mold, and subsequently it is not easily possible to change the stringing subsequently.
The material for the supports or crosspieces, brought in additionally in a not exactly controllable way, further leads to an increase in weight and to a final, established swing behavior or to considerable limitations with respect to the balance of the racket.
To avoid such ribs, crosspieces, supports and connections, previously there was essentially known an especially labor-intensive process in which partially set material in the form of sheets was used. For the production of strong, lightweight objects, such as for the production of racket frames, such material must be molded around a core structure, and this core structure is to expand during the molding operation to yield a satisfactory stabilization. Such an expandable core can be made of an expandable plastic or be used as inflatable tube, e.g., as silicon rubber tube, which can, if necessary, be removed subsequently from the molded object. Such a procedure is naturally exceedingly labor-intensive, especially because the thermosetting resins used must be kept for several minutes at a suitably high temperature to guarantee the setting and because, for mass production, for this reason a large number of molds is necessary.